Steam generator having a minimum load firing system



Sept. l0, 1968 R. I ITWINOFF STEAM GENERATOR HAVING A MINIMUM LOAD FIRING SYSTEM Filed June 27.

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ATTOZE VS United States Patent O 3,400,559 STEAM GENERATOR HAVING A MINIMUM LOAD FIRING SYSTEM Richard Litwinolf, Winterthur, Switzerland, assignor to Sulzer Brothers Ltd., Winterthur, Switzerland, a corporation of Switzerland Filed June 27, 1966, Ser. No. 560,446 Claims priority, application Switzerland, July 1, 1965, 9,240/ 65 11 Claims. (Cl. 122-479) ABSTRACT OF THE DISCLOSURE A minimum load firing system is mounted centrally within the combustion chamber and the main firing system. The llame of -the minimum load firing system is varied to produce a variable load especially during shutdown of the main firing system. This flame enables a uniform heat across the heating surfaces to be maintained and also allows rapid start up of the main firing system after shutdown.

The invention relates to a steam generator having a minimum load firing system. More particularly, the invention relates to a steam generator having a minimum load rng system for operation during low demand periods.

Steam generators have heretofore been known wherein the combustion chambers have been incorporated with minimum load firing systems in addition to a main tiring system for the purpose of operating the generators at low demand load periodsof time. The minimum load firing systems-.have been particularly useful when the ysteam generator in a` steam-power plant used for compensating the base load requirements has been operated overnight at a low load.,In such cases, the minimum load firing system of the plant has been operated with a small firing rate which is smaller than the smallest firing rate of the main tiring system. However, it has been shown that'despite the small rate of firing of the minimum load firing system, pipe damage has occurred as the result of local overheating, becausethe flow rate of the working medium in -the pipes has been kept small in accordance with the firing rate.

Accordingly, it is an object of the invention to provide a steam generator. with minimum load tiring system which distributes heat uniformly over the heating surfaces of a combustion chamber. l

It is another object of the invention to provide Ia steam generator with `a minimum load firing system having a means to vary the height of llame.

It is another object of the invention to provide a steam generator with .a minimum load firing system which assists a rapid start up of the main tiring system of the steam generator.

It is another object of the invention to efficiently regulate the temperature of a steam generator at minimum operating loads in order to vary the heat absorption of the superheater heating-surfaces of the steam generator. l Generally, the invention incorporates a llame height varying means into a minimum load firing system of a steam generator having a main firing system located within acombustion chamber. The llame height varying means functions such as to redirect the flame produced 'by a fixedly mounted minimum load firing system to achieve a different height relative to a heating surface in the combustion chamber of the steam generator or to move a minimum load firing system relative to a heating surface in the combustion chamber.

By varying the flame height of the minimum load firing system the invention avoids any damage to the pipes of the ice steam generator by overheating. Further, the invention enables the llame height to be varied continuously or intermittently during a minimum load period such as at night and consequently permits the heat provided by the minimum load firing system to be distributed as uniformly as possible over the heating surfaces in the combustion chamber during the entire period. Moreover, the invention assists restarting of the steam generator after minimum load operation so that the process can take place more rapidly. Finally, the invention makes it possible to improve the temperature regulation of the steam generator at minimum operating loads by varying the llame height of the minimum load firing system for the purpose of modifying the heat absorption of the superheater and/or of the intermediate superheater heating surfaces.

These and other objects and advantages of the invention will become more apparent from the following detailed description and `appended claims taken in conjunction with the accompanying drawings in which:

FIG. l illustrates a diagrammatic longitudinal section through a steam generator with a minimum load firing system permanently mounted in the center of the combustion chamber;

FIG. 2 illustrates a longitudinal section through the minimum load firing system to a scale larger than that of FIG. 1;

FIG. 3 graphically illustrates the firing system output in relation to a base of the steam generator load;

FIG. 4 illustrates a diagrammatic longitudinal section through a steam generator embodied with a minimum load firing system moveably disposed in the center of the combustion chamber;

FIG. 5 illustrates a diagrammatic longitudinal section through a steam generator combustion chamber embodied with a minimum load firing system capable of swivelling; and

FIG. 6 illustrates in diagramamtic form a regulator system for the steam generator according to FIG. 5.

Referring to FIG. 1 a steam generator includes a combustion chamber 1 of rectangular cross section lined with pipes 2 which represent an evaporator heating surface and provided with re wall heating surfaces 46 in the upper part through which high pressure steam or partially expanded steam ows so that the heating surfaces 46 act as superheaters or intermediate superheaters. In addition, heating surfaces 14, 15, 16 and 17, which may act as further heating surfaces for superheaters, intermediate superheaters and economisers, are mounted in the flue 11 of the steam generator in a conventional manner. The main ring system is Kcomposed of burners 3 which are disposed in the four corners of the combustion chamber 1. A minimum load firing system 5 is lixedly mounted and located in the center of the combustion chamber 1 and is supplied with fuel, such as oil, through a pipeline 6 and air through a pipeline 7. Each pipeline 6 and 7 is provided with valves 18 and 19 respectively to control the flow of fuel and air therethrough.

A pipe coil 4 through which a coolant flows surrounds the part of the minimum load firing system 5 which eX- tends into the combustion chamber. The pipe coil 4 is provided with Water as the coolant which is obtained from the Working medium circuit of the steam generator through pipe coils connected in parallel with the economiser heating surface. The coolant enters the pipe coil 4 through an inlet 8 and exits through a discharge 9 for circulation through the pipe coil.

In addition, the minimum load firing system 5 is supplied with flue gas from a flue gas return line 10 connected to the flue 11 which terminates in the ring system 5. The flue gas is supplied to the minimum load liring system by means of a blower 12 disposed in the pipeline 10 and the quantity of flue gas is adjusted by means of a Valve 13. A valve 33 in the minimum load tiring system distributes the returned ilue gas in the form of a curtain over the tire.

Referring to FIG. 2, the minimum load firing system is designed with the upper end of the combustion air pipelines 7 in the form of a pipe shell surrounded by the cooling pipe coil 4. The end section of the ue gas return line 10 is formed with a restriction and located within the pipe shell 7. The restricted end of the ue gas return line 10V is joined to the end of the pipe shell 7 through a coneshaped shell in which four uniformly distributed burners 22 are mounted circumferentially. Each burner 22 is surrounded by concentric air diffusers 23 which may be adjustable. The fuel supply line 6 extends into the `annular chamber between the pipe shell 7 and the flue gas line 10 and terminates in a ring main 25 from which a supply line 24 is branched off to each burner 22. The upper restricted end of the flue gas line 10 houses a mushroom valve 33 having a stem 20 which is guided in two bearings 31 and 32 for closing the gas line 10. The bearings 31, 32 are retained by four radial walls 30 which are secured to the pipeline 10 at a position in front of the restriction thereof. A spring 34 is provided between the bearing 32 and the free end of the valve stem 20 to urge the mushroom valve 33 towards the pipeline 10. Radial pipes 36 which terminate in a ring duct 35 extending around the internal circumference of the pipe shell 7 branch off from the ue gas line 10 approximately at half the height of the pipe shell 7 which projects into the combustion chamber. In the zone of the ring duct 35, the pipe shell 7 is provided with holes 37 through which the flue gas can escape and then flow upwards past the pipe coil 4.

When the steam generator operates at normal load only the corner burners 3 of the main tiring system will be in operation While the minimum load firing system will be shut down. The minimum load firing system is protected during this period by the coolant which flows through the pipe coil 4. If necessary, a further small amount of cooling air may be ducted in the direction of the arrow 27 through the annular space between the pipe shell 7 and the flue gas line 10 to provide further protection. After passing through the air ditfusers 23, such cooling air functions as a secondary air supply in the combustion chamber 1. During this kind of operation, that is, with the burners 22 of the minimum load firing system shut down, flue gas may flow for cooling purposes through the line of the minimum load tiring system.

Referring to FIG. 3, only the main firing system remains in operation during a load reduction down to approximately 35% of the normal steam generator load (area B). When this load level is reached only the uppermost row of corner burners 3 are in operation thereby substantially reducing the output of the main firing system. At the same time the minimum load ring system 5 is ignited. A further reduction of the load causes the main firing system to be completely shut down so that only the minimum load tiring system 5 is in operation. The output of the minimum load firing system 5 can be reduced, according to the area A in FIG. 3, by varying the supply of fuel with the aid of the valve 18. The flue gas flowing through the line .10 opens the mushroom valve 33 and distributes itself over the flame of the burner 22. The height of this flame will vary, depending on the position of the mushroom valve 33, so that the flue gas curtain which spreads over the flame alters its position so that the streaks of hot gas which are simultaneously emitted by the minimum load tiring system 5 are prevented from excessively heating individual tire walls or parts thereof. The spring 34 urging the mushroom valve 33 towards the pipeline 10 ensures an adequate minimum exit velocity of the flue gas even if only a very small amount of flue gas is returned.

According to another embodiment of the invention, the

position of the mushroom valve 33 is varied either manually or in relation to a temperature measured in the end zone of the combustion chamber. In the latter case, a temperature measuring point 95 is provided, for example, on one of the fire wall heating surfaces 46.

Referring to FIG. 4, a steam generator provided with front burners 26 which function as the main firing system and designedyfor a high sub-critical pressure has a relatively .small evaporator heating surface. This evaporator heating surface is formed bypipes 40 and 41 which line a combustion chamber 45 and terminate in a liquid separator 42. The steam to be superheated reaches a pipeline wall which lines the upper part of the combustion chamber 45 as well as a pipeline wall 43 which covers the rear wall of the combustion chamber. Fire wall heating surfaces 46 which function as heating surfaces for the intermediate superheater are provided in the upper part of the combustion chamber 45. A minimum load ring system 21 which is vertically adjustable with the aid of telescoping pipes and which can also be completely withdrawn from the combustion chamber during operation of the main firing system is provided in the center of the combustion chamber cross section. A fuel supply line in the form of a flexible hose 47 supplies fuel to the minimum load firing system 21 while combustion air ows upwards in the direction indicated by the arrows 48 within the telescoping pipes. The amount of fuel and the amount of air are adjustable as above.

Since the minimum load firing system is not located within the combustion chamber during operation of the main tiring system, it is not necessary to provide protective devices such as the coolant pipe coils 4 of the above examples in accordance with FIGS. 1 and 2. Further, the minimum load tiring system 21 is ignited only after the lower row of the burners 26 of the main firing system is shut down.

Referring to FIG. 5, a main firing system of a modied form of steam generator consists of a double U firing system in which burners 49 are disposed on so-callled banks above a combustion chamber 38. The flames of the burners 49 are initially directed downwardly and then reversed to meet in the center of the combustion chamber 38. The combustion chamber 38 is lined in a manner similar to that shown in FIG. 1 and the lire wall heating surfaces are positioned in the upper part of the combustion chamber. The minimum load tiring system 39 is supported by an arm 80 which is located on one side of the combustion chamber 38 for swivelling around a horizontal axis 81. The vertical position of the flame of the minimum load firing system 39 can thus be varied by appropriate adjustment of the swivelable arm 80. When the burners 49 of the main ring system are in operation, the minimum load firing system 39 is shut down and the arm is swung into a chamber 82 which is located outside the combustion chamber for housing therein. The arm 80 when positioned in the chamber 82 is therefore adequately protected from the main ring system. Before the main tiring system is shut down when the load falls below a certain level, the arm 80 is moved into the combustion chamber 38 and the minimum load firing system 39 is ignited. Thereafter, the arm is moved further into the combustion chamber depending on the desired llame height.

Referring to FIG. 6, the steam generator of FIG. 5 is provided with an evaporator heating surface 50, an interconnected superheater heating surface 52 and a liquid separator 51 mounted in the linebetween the heating surfaces 50 and 52. Superheated high pressure steam passes through a line 60 to a load which is not shown. The end of the superheater 52 has a temperature measuring element 83 connected thereto which, by acting through a regulator 84, adjusts a valve 85 which is provided in a feed water line 86 connected on the inlet side of the evaporator heating surface 50. The feed water line 86 terminates at 62 in the superheater 52 and the amount of water fed into the system is adjusted by means of the regulator 84 and the valve S5 so that the eXit temperature of the steam is maintained at a desired value. Feed water is also supplied to the evaporator heating surface 50 by means of a feed pump 53 which is regulated by a regulator 77 which in turn is connected to a diaphragm 55 provided to measure the feed water flow rate. The amount of fuel to be supplied to the burners 49, of which only one is shown in the drawing, is adjusted with the aid of a valve 56 which is controlled by a regulator 87 connected to a diaphragm 88 which measures the fuel flow rate. The amount of fuel to be supplied to the minimum load firing system 39 is adjusted by a valve 57 which, in the same way as the valve 56, is controlled by a regulator 89 connected to a diaphragm 90 which measures the fuel flow rate. A pressure sensing element 63 which acts through a regulator 64 on to a load transmitter 65 is connected to the steam line 60. The load transmitter 65 is connected by means of signal lines 66 and 6'7 to the fuel How rate regulator 87 of the main firing system and the fuel flow rate regulator 89 of the minimum load firing system to provide the regulators 87 and 89 with set-value signals. In addition, in order to regulate the height of the fla-me of the minimum load ring system 39 the load transmitter 65 is connected by a set-value signal line 68 to a regulator 69 which is modulated by a temperature sensing element 91 provided upstream of the feed water inlet position 62 on the superheater 52 and which is connected by a signal line 79 to a servo-motor 70 which swivels or deflects the arm 80 of the minimum load firing system 39. The arm 80 with the minimum load firing system is so deflected that it is raised when the steam temperature upstream of the feed water inlet position 62 is too low and vice versa.

The regulator 77 which controls the amount of feed water supplied receives a signal in a conventional manner from a regulator 76 via a line 92. The signal received depends on the steam temperature of the section of the evaporator heating surface which is operated at superheat temperature. The steam temperature is measured by a temperature sensing element 75 connected to the regulator 76; the regulator being provided with a set-value obtained from the -load transmitter 65 (not shown). The temperature sensing element 75 is also connected with a comparator element 78 in which the temperature signal is compared with a limiting value provided through a line 93. The comparator element 78 is connected to the signal line 79 to provide an overriding signal for the servomotor 70 if the steam temperature in that section of the evaporator heating surface which is operated at superheat should risel above the limiting value. In such a case, the overriding signal acts on the servomotor 70 to cause the ame of the minimum load firing system 39 to be displaced in a downward direction.

Similarly, the ame heights of the minimum load ring systems in the steam generators according to FIGS. 1 and 4 can also be regulated in relation to temperature.

Having thus described the invention, it is not intended that it be so limited as changes may be readily made therein without departing from the scope of the lnvent'ion. Accordingly, it is intended that the subject matter described above :and shown in the drawings be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In combination with a steam generator having a combustion chamber, `a main firing system in said combustion chamber and a minimum load firing system fixed- 1y mounted centrally in said combustion chamber, said minimum load firing system including means for producing a llame to operate the steam generator during shut down of the main firing system; means for varying the llame height of said minimum load ring system.

2. The combination as set forth in claim 1 wherein said llame height varying means includes a ilue gas return line means for supplying flue gas to said minimum load tiring system, and an element mounted in said minimum load ring system in the path of flow of the supplied flue gas for distributing the supplied flue gas over said llame.

3. The combination as set forth in claim 2 wherein said element distributes the supplied flue gas in the form of a curtain.

4. The combination as set forth in claim 2 wherein said element is a mushroom valve.

5. The combination as set forth in claim 2 which further comprises spring means for biasing said element in `a direction opposite the flow of the supplied flue gas.

6. The combination as set forth in claim 1 wherein said means for producing a flame includes a plurality of circumferentially disposed burners, and said ame height varying means includes a pipe disposed centrally of said burners for supplying flue gas to said minimum load firing system and a mushroom valve slidably mounted in said pipe in the path of ow of the supplied ue gas for distributing the supplied llue gas over said burners.

7. The combination as set forth in claim 1 wherein said means for varying the llame height includes a vertically `adjustable means supporting said minimum load firing system centrally of said combustion chamber for moving said minimum load ring system with respect to the interior of said combustion chamber.

S. The combination asset forth in claim 7 wherein said vertically adjustable means include a plurality of telescoping pipes.

9. The combination as set forth in claim 1 wherein said means for varying flame height includes a regulating device responsive to a temperature measured in a zone of the end of said combustion chamber to vary said flame height.

10. In combination with a steam generator having a combustion chamber, a main ring system in said combustion chamber and a minimum load firing system in said combustion chamber, said minimum load firing system including means for producing a llame t0 operate the steam generator during shut down of the main firing system; means for varying the flame height of said minimum load firing system, said means for varying the llame height including la swivelly mounted arm supporting said minimum lload firing system for swivelling about a horizontal axis, said varm being mounted to a side of said combustion chamber.

11. The combination as set forth in claim 10 which further comprises a chamber outside of said side of said combustion chamber for housing said arm therein when said arm is in a downwardly directed vertical position.

References Cited UNITED STATES PATENTS 2,100,190 11/1937 Jackson 122-479 X 2,367,193 1/1945 Blizard 122-479 2,851,018 9/1958 Heller.

3,003,479 10/1961' Bock et al 122--1 3,160,146 12/1964 Kuhner 122-479 X CHARLES I. MYHRE, Primary Examiner. 

